A system pulling 500w is going to still pull 500w no matter what size PSU you use.
500w will be pulled from 1200w the same as 500w will still be pulled from 600w.
If your looking for money saving options, the higher 80PLUS efficiency the better. However, also bare in mind the 80PLUS Gold/Platinum PSU's hold a hefty price tag over Bronze/Silver rated and due to the extra cost you will never see any savings in terms of your electricity bill within the PSU's "lifetime".
PSU's best efficiency range is usually when they are operating about 60% load. So if you have a system constantly pulling 500w, then about a 750w PSU would be recommended. Again though, be aware your system wont always been running the same wattage, 500w is likely to be what the system with use when every component is at 100% usage which unless benchmarking - is likely to never happen.